The present invention relates to the art of magnetic resonance cine imaging. It finds particular application in conjunction with cine imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with angiography, circulatory, cardiac quantitative flow, and other examinations in which flowing fluid or moving tissue is imaged.
Cine images have commonly been acquired using field echoes. Field echoes permit a rapid, e.g. 5-10 msec, repetition rate. In cardiac imaging, about 64 to 100 sequence repetitions can be made in a typical 600 to 1000 msec. cardiac cycle. If the view from each sequence repetition in a common cardiac cycle is allocated to a successive frame, then the cine sequence produces about 64 to 100 frames per cardiac cycle with a resolution of about 10 msec.
Optimal cardiac gated cine images are achieved if all the views are collected within a single breath hold. Collecting all the views within a single breath hold eliminates the motion artifacts attributable to respiratory motion. A single breath hold is typically 16-20 heartbeats. Acquiring only one view per frame image in each cardiac cycle of a single breath hold would limit each frame image to 16-20 views. To increase the number of views per frame, data acquisition could be continued for several breath holds. However, the heart and surrounding tissue move with each respiratory cycle and typically do not return accurately to the same position in subsequent breath holds.
In one technique described in "Cineangiography of the Heart in a Single Breath Hold with a Segmented TurboFLASH Sequence", Radiology, Vol. 178, pp. 357-360, Atkinson and Edelman (RSNA, 1991), 128 lines or views of k-space were grouped into 8 segments of 16 views each. The resultant magnetic resonance echoes of each cardiac cycle were grouped into 16 groups of 8 views each. The number of groups were dependent on the patient's heart rate. The 8 views within each group of 16-20 consecutive heart beats were differently phase encoded and processed as 128 views (16.times.8) of the same frame image. In this manner, multi-frame images of the cardiac cycle, each with 128 views per image, were generated. More specifically, in the first heart beat, views or lines 1, 17, 33, etc. were collected; in the second heart beat, lines 2, 18, 34, etc. were collected; and so forth. The raw data from these sets was combined or interleaved to form the 128 view data set for reconstruction into each of the multiple frames.
One of the drawbacks of the Atkinson and Edelman segmentation of k-space was that the resultant frames suffered from blurring. The present inventor has recognized that this blurring is attributable to the acquisition of the central views, the views with the most signal power and image information, at different times in subsequent cardiac cycles. In the above-illustrated 8 segment, 16 heart beat sequence, the central views in each group alternated between two different time displaced points in the cardiac cycle.
A fast spin echo imaging technique which acquires data over a wide range of echo times is described in "Shared Data Dual Echo in Fast Spin Echo Imaging" of R. Scott Hinks and Steve Einstein, SMRM Book of Abstracts, page 1011, (1991). Images were acquired corresponding to multiple effective echo times such that each had different properties, e.g. a different T.sub.2 weighting. In the Hinks and Einstein technique, each excitation was followed by a plurality of echoes. The central low frequency views were taken in the early echoes and the higher frequency views were taken in the later echoes. To expedite collection, the central, low frequency views, e.g., the central 32 views of a 256 view study were collected twice. The remaining, higher frequency views were each collected once and shared with two lower frequency groups. This produces a significant difference in the T.sub.2 weighting between high and low frequency views and a low signal to noise ratio for the high frequency views as compared to a normal spin echo scan.
In parent application 07/874,807, groups of sequential excitations, e.g., 3 excitations in a row, produce 3 echoes in a row which are all used to produce views of a common image. The central most, lowest frequency views are in the center of the group and the higher frequency views are at the beginning and end of the group. By grouping three consecutive echoes into each image, images with an effective temporal resolution of about 30 msec. are achieved with one third as many repetitions as when each echo is processed as a view of a different time displaced frame image. Similar results are achieved for groupings of different sizes.
The present invention provides a new and improved field echo cine technique in which the resultant cine images have more views per frame with better temporal resolution.